diff --git a/ihatemoney/forms.py b/ihatemoney/forms.py
index 6800df10..27e438b9 100644
--- a/ihatemoney/forms.py
+++ b/ihatemoney/forms.py
@@ -325,7 +325,7 @@ class BillForm(FlaskForm):
     original_currency = SelectField(_("Currency"), validators=[DataRequired()])
     external_link = URLField(
         _("External link"),
-        default=None,
+        default="",
         validators=[Optional(), URL()],
         description=_("A link to an external document, related to this bill"),
     )
diff --git a/ihatemoney/tests/api_test.py b/ihatemoney/tests/api_test.py
index 9a2d2149..1365fa02 100644
--- a/ihatemoney/tests/api_test.py
+++ b/ihatemoney/tests/api_test.py
@@ -539,7 +539,7 @@ class APITestCase(IhatemoneyTestCase):
                 "amount": expected_amount,
                 "date": "2011-08-10",
                 "id": id,
-                "external_link": None,
+                "external_link": "",
                 "original_currency": "XXX",
                 "converted_amount": expected_amount,
             }
@@ -834,7 +834,7 @@ class APITestCase(IhatemoneyTestCase):
             "amount": 25.0,
             "date": "2011-08-10",
             "id": 1,
-            "external_link": None,
+            "external_link": "",
             "converted_amount": 25.0,
             "original_currency": "XXX",
         }